<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>单位</title>
    <style>
        html{
            font-size: 20px;
        }
        .box{
            height:300px;
            width: 300px;
            background-color: green;
        }
        .box1{
            height: 50%;
            width: 50%;
            background-color: red;
        }
        .box3{
            font-size: 30px;
            /* width: 10em;
            height: 10em; */
            width: 10rem;
            height: 10rem;
            background-color: blue;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="box1">
        </div>
    </div>
    <div class="box3">
    </div>
</body>
</html>